The present invention relates to a semiconductor memory device for reading charges stored in a capacitor in a memory cell and a data reading method thereof and, more particularly, to a dynamic random access memory (DRAM) or a ferroelectric memory.
FIG. 1 shows a circuit arrangement of a basic ferroelectric memory as an example of a semiconductor memory device for reading charges stored in a capacitor in a memory cell. This circuit comprises memory cells MC, dummy cells DMC, a sense and rewrite amplifier (sense amplifier) 18, a word line i 19, a word line (i+1) 20, a dummy word line a 21, a dummy word line b 22, a plate line i 23, a plate line (i+1) 24, a dummy plate line a 25, a dummy plate line b 26, a pair of bit lines BL and BL as a differential pair, transistors 300 and 301 for selecting a column, a column selection line 302, and a pair of common read data lines DL and DL. The memory cells MC have ferroelectric capacitors 10 and 11 and selection transistors 14 and 15, respectively. The dummy cells DMC also have ferroelectric capacitors 12 and 13 and selection transistors 16 and 17, respectively.
In the above arrangement, as the direction of the electric field in the ferroelectric memory and the direction of polarization (these two directions match), the direction from the plate line to the bit line is defined as a positive direction. In the ferroelectric memory, the bit lines have a level difference in accordance with direction of polarization of a ferroelectric capacitor storing data. The read operation will be described in detail with reference to the timing chart of FIG. 2. The bit line is precharged to 0 (V) in advance. A word line connected to a cell to be selected is selected, and in this state, a plate line is selected. After the potentials of the pair of bit lines BL and BL change, the sense and rewrite amplifier 18 is activated to set one of the bit lines BL and BL at high level and the other at low level. In the circuit shown in FIG. 1, when the ith word line 19 is selected, the dummy word line 21 is selected. When the (i+1)th word line 20 is selected, the dummy word line 22 is selected.
Assume that the power supply voltage is 3 (V), and the potentials of the selected plate line and dummy plate line become 3 (V) at maximum. Also, assume that the maximum value of the potentials of the selected word line and dummy word line is boosted to a voltage (e.g., 4.5 (V)) for compensating for a drop in threshold voltage by a cell selection transistor and dummy cell selection transistor such that the high potential of the pair of bit lines BL and BL is transmitted to the capacitor.
When polarization in the capacitor in the selected memory cell is directed upward (from the plate line side to the bit line side), polarization inversion does not occur because the direction of polarization matches that of the electric field. In this case, since the amount of charges removed from the cell is small, the level of the bit line is low. To the contrary, when polarization is directed downward (from the bit line side to the plate line side), polarization inversion occurs because polarization and the electric field are directed in opposite directions. In this case, since the amount of charges removed from the cell is large, the level of the bit line is high. Hence, when the areas of the ferroelectric capacitors 12 and 13 in the dummy cells are set to generate the intermediate level between the bit line level when polarization inversion occurs and that when polarization inversion does not take place, the level difference between the bit lines BL and BL can be sensed by the sense and rewrite amplifier 18.
The scheme of sensing the bit line level while setting the plate line at high level, as shown in FIG. 2, is called "during plate pulse sensing" for the descriptive convenience. Such a data read operation is disclosed in U.S. Pat. No. 4,873,664. This patent describes that the areas of the ferroelectric capacitors 12 and 13 in the dummy cells are set to be twice those of the ferroelectric capacitors 10 and 11 in the memory cells to generate the intermediate potential. The dummy capacitor in the dummy cell must be biased such that the dummy cell does not operate in the polarization inversion region, i.e., polarization is always directed upward (from the plate line to the bit line).
To clarify the problem of the conventional circuit, the bit line level to be sensed will be described using a graphic solution of the hysteresis characteristics of the ferroelectric capacitor. The positive direction is defined as a direction from the plate line to the bit line. Let V.sub.f be the voltage applied to the ferroelectric capacitor. Referring to FIG. 3, while the potential relationship is changing from (a) to (b), the amount QB of charges in the bit line is kept unchanged because the bit line is not charged/discharged. This situation can be represented as: EQU Q.sub.B =+C.sub.B.times.0-P(0)A=+C.sub.B (3-V.sub.f)-P(V.sub.f)A (1)
where A is the area of the ferroelectric capacitor, and C.sub.B is the parasitic capacitance of the bit line.
Equation (1) can be rewritten as: EQU P(V.sub.f)=P(0)+C.sub.B (3-V.sub.f)/A (2)
Based on equation (2), the voltage V.sub.f applied to the ferroelectric capacitor in during plate pulse sensing is given by the coordinate value on the abscissa at the intersection between the hysteresis characteristics P=P(V.sub.f) of the ferroelectric capacitor and P=P(0)+C.sub.B (3-V.sub.f)/A. Therefore, the voltage V.sub.f when polarization inversion occurs from polarization directed downward (high level), the voltage V.sub.f when polarization inversion does not occur from polarization directed upward (low level side), and the voltage V.sub.f by the dummy cell are obtained as shown in FIG. 4. Each bit line potential is also obtained on the basis of equation (3) as shown in FIG. 4. EQU V.sub.B =3-V.sub.f (3)
According to the hysteresis characteristics of the cell, the gradient of P(V.sub.f)A with respect to the voltage V.sub.f is proportional to the electrostatic capacitance of the cell. Pieces of information stored in the bit lines BL and BL belonging to the selected column 302 are sent to the common read data lines DL and DL through the transistors 300 and 301 for selecting the column, respectively.
Generally, a sense amplifier is constituted by a flip-flop including p-channel MOS (PMOS) transistors 217 to 219 and n-channel MOS (NMOS) transistors 220 to 223, as shown in FIG. 5. The PMOS transistor 217 and NMOS transistor 223 serve as a power switch of the flip-flop. When a gate 206 of the Transistor 217 is at level "0", and a node 203 of the transistor 223 is at level "1", the flip-flop is activated to start the sense operation. Normally, to decrease the through current flowing from a power supply T to a ground point 2 through the PMOS and NMOS transistors in the flip-flop, the ON timings of the transistors 217 and 223 as a power supply switch are shifted. When common read data lines are to be precharged to the power supply voltage, the switching speed of the data line DL or DL is determined on the basis of the change speed from high level to low level. Therefore, the sense amplifier drives the bit line from the NMOS transistor side.
As is apparent from FIG. 4, the electrostatic capacitance of the dummy cell is about twice that of the cell capacitor in the cell on the low level side. For example, assume that before the start of the sense operation, the potential of the bit line BL on the low level side is 1.0V, and the potential of the bit line BL on the dummy cell side is 1.1V. Since the potential of the gate (bit line BL) of the NMOS transistor 221 for reducing the potential of the bit line BL in the sense amplifier is slightly lower than that of the gate (bit line BL) of the NMOS transistor 220 for reducing the potential of the bit line BL, the current driving capability of the transistor 220 for reducing the potential is higher than that of the transistor 221. In addition, since the electrostatic capacitance of the bit line BL viewed from the sense amplifier is larger than that of the bit line BL because of the large electrostatic capacitance of the dummy capacitor of the bit line BL, the potential of the bit line BL decreases at a higher speed than that for the bit line BL. That is, the potential difference between the bit line BL and BL increases. Immediately after this, the PMOS transistor 217 is turned on. Since the gate potential of the PMOS transistor 219 for increasing the potential of the bit line BL is lower than that of the PMOS transistor 218 for increasing the potential of the bit line BL, the current driving capability of the transistor 219 for increasing the potential is higher than that of the transistor 218. Dn the other hand, since the electrostatic capacitance of the bit line BL viewed from the sense amplifier is larger than that of the bit line BL, the potential of the bit line BL increases at a higher speed than that for the bit line BL. When the current driving capability difference between the transistors 219 and 218 is smaller than the electrostatic capacitance difference between the bit lines BL and BL, the potential of the bit line BL may increase at a higher speed than that for the bit line BL, resulting in an erroneous operation.
Since the cell capacitor of the cell on the high level side draws a locus continued from polarization, the difference from the electrostatic capacitance of the dummy capacitor cannot be uniquely defined. In FIG. 4, the electrostatic capacitance of the cell on the high level side is much larger than that of the dummy capacitor. However, the electrostatic capacitance of the cell largely changes depending on the characteristics of the cell or the magnitude of the parasitic capacitance of the bit line.
For example, assume that before the start of the sense operation, the potential of the bit line BL on the high level side is 1.2V, and the potential of the bit line BL on the dummy cell side is 1.1V. As shown in FIG. 4, when the electrostatic capacitance of the cell on the high level side is larger than that on the dummy cell side, and the sense amplifier is driven from the NMOS transistor side, the potential difference between the bit lines BL and BL increases, as described above. When the PMOS transistor is subsequently driven, a locus indicated by the broken line in FIG. 4 is obtained. Since the electrostatic capacitance of the bit line BL viewed from the sense amplifier is smaller than that of the bit line BL, the potential of the bit line BL increases at a higher speed than that for the bit line BL, so data can be properly read.
Conversely, when the electrostatic capacitance of the cell on the high level side is smaller than that on the dummy cell side, and the sense amplifier is driven from the NMOS transistor side, the potential difference between the bit lines BL and BL decreases, as described above, and the potential relationship may be reversed.
As described above, not only the potential difference between the bit lines BL and BL before sensing but also the unbalance in electrostatic capacitance influences on the sense sensitivity, so data cannot be properly read in some cases.
A scheme of temporarily increasing the potential of a selected plate line to 3 (V) and then reducing the plate potential and sensing the potential is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 1-158691 or Integrated Ferroelectrics, Vol. 4, pp. 134-144. FIG. 6 is a timing chart of this scheme. In the polarization inversion operation, it is actually supposed that a phenomenon in which the direction of polarization in the domain is actually reversed and a non-polarization inversion phenomenon (this can be regarded as relatively linear polarization due to electronic or ionic polarization) overlap each other. In this scheme, even when non-polarization inversion varies, this can be canceled by returning the plate potential to 0 (V), so variations in bit line to be sensed can be decreased. Hence, even the non-polarization inversion region varies in the reliability test, the influence can be minimized. This sensing scheme will be referred to as "after plate pulse sensing" hereinafter.
After plate pulse sensing will be analyzed below. The bit line potential in after plate pulse sensing is obtained by a graphic solution. FIG. 7 shows a change in charges in the ferroelectric capacitor or bit line capacitance in this scheme. The change from state (a) to state (b) in FIG. 7 is the same as in during plate pulse sensing. In the change from state (b) to state (c) as well, the bit line is not charged/discharged at all. For this reason, the amount of charges in the bit line in states (a) and (c) is kept unchanged. Hence, equation (4) holds: EQU Q.sub.B =+C.sub.B.times.0-P(0)A=-C.sub.B V.sub.f -P(V.sub.f)A (4)
Equation (4) can be rewritten as EQU P(V.sub.f)=P(0)-C.sub.B V.sub.f /A (5)
Based on equation (5), the voltage V.sub.f applied to the ferroelectric capacitor in after plate pulse sensing is given by the coordinate value on the abscissa at the intersection between the hysteresis characteristics P=P(V.sub.f) of the ferroelectric capacitor and P=P(0)+C.sub.B V.sub.f /A. Therefore, the voltage V.sub.f when polarization inversion occurs from polarization directed downward (high level) and the voltage V.sub.f when polarization inversion does not occur from polarization directed upward (low level) in after plate pulse sensing are obtained as shown in FIG. 8. Each bit line potential is also obtained on the basis of the equation below as shown in FIG. 8.
As is apparent from FIG. 7, even in after plate pulse sensing, the electrostatic capacitance of the cell largely changes between high level and low level. The capacitances of the bit lines BL and BL do not balance in sensing independently of the electrostatic capacitance of the dummy cell. Especially, in after plate pulse sensing, the potentials of the bit lines BL and BL are lower than those in during plate pulse sensing. For this reason, even when the sense amplifier is driven from the NMOS transistor side, the NMOS transistor is not turned on immediately after the start of the sense amplifier operation. Hence, the potential difference between the bit lines BL and BL cannot be obtained unless the PMOS transistor is driven. For example, assume that the electrostatic capacitance of the dummy cell is at the intermediate level between the electrostatic capacitances of the bit lines BL and BL. In this case, like during plate pulse sensing, the potential difference between the bit lines BL and BL becomes small independently of high or low level due to the unbalance in electrostatic capacitance between the bit lines BL and BL, and finally, the potential relationship may be reversed.